Whooping cough has sickened a rising number of children in northern and eastern Maine, according to the Maine Center for Disease Control and Prevention.

The highly contagious respiratory infection has hit Washington County the hardest, at a rate of 115 cases per 100,000 residents, six times the statewide average. Six other counties — Aroostook, Knox, Lincoln, Oxford, Penobscot and Waldo — also reported high rates of whooping cough, according to the Maine CDC.

Health officials say the cyclical nature of the disease coupled with a lack of vaccinations are likely to blame.

Statewide, health providers have reported 254 cases of whooping cough this year, according to a health advisory issued by the Maine CDC on Aug. 4. The majority of cases involve people between the ages of 7 and 19.

Maine recorded 332 cases last year, following a historic spike of 737 cases amid a national outbreak in 2012 — the highest since at least the 1960s.

Whooping cough, or pertussis, tends to cycle, with peaks every three to five years, Maine CDC Director Dr. Sheila Pinette explained. The disease broke out in Bangor toward the end of 2012, making its way to York and Cumberland counties last year, then moving up the midcoast, she said.

Health officials noted an uptick of whooping cough toward the end of the school year and hoped for a dropoff after classes let out for the summer, Pinette said. But the disease is appearing at daycares and at several summer camps, she said.

“We need to immunize and practice good hand-washing and respiratory etiquette,” Pinette said.

Pertussis is preventable with a vaccine, though some vaccinated children still catch the disease. The vaccine wears off over time, but it slashes the risk of catching and spreading the disease and generally results in a more mild case, according to health officials.

A previous formulation of the vaccine — phased out in the U.S. during the 1990s — was more effective but carried serious side effects, including high fevers.

While initial immunization rates are high among children, some miss the required Tdap booster shot, which covers tetanus, diphtheria and pertussis, administered at age 11 or 12. The booster vaccination, recommended before children return to school, takes full effect in about two weeks.

About 95 percent of Maine kindergartners were vaccinated against pertussis during the 2012-13 school year, according to the U.S. CDC. While that figure seems high, vaccines are most effective when the entire population is immunized. Even a slight dip in vaccination rates can greatly increase the chances of an outbreak.

More than 4 percent of Maine kindergartners — or 620 students — were exempted from required vaccines, including for pertussis, in 2012-13. In the majority of cases, parents cited “philosophical objections” to vaccines, rather than for medical reasons.

Maine is among the top 10 states with the highest vaccine opt-out rates, U.S. CDC figures show.

According to 2012 data, some of the counties hit hardest by pertussis this summer actually have relatively high immunization rates. In Washington County, 86 percent of 2-year-olds are up to date on their shots, along with 89 percent in Aroostook County.

Hancock, Waldo and Lincoln counties had the lowest rates.

The unrelenting cough caused by pertussis can prove life-threatening to infants. Children too young for the shots or who haven’t built up enough immunity often catch whooping cough from a loved one, who never was vaccinated or failed to stay current with booster shots.

As of this month, Maine has recorded 11 cases of whooping cough in children younger than 1 year old, Pinette said. All have survived the disease, but the majority were hospitalized, she said.

The first symptoms of whooping cough appear similar to the common cold, including a sore throat and runny nose, followed by a severe cough developing after a few weeks. People with whooping cough usually spread the disease by coughing or sneezing while in close contact with others.

The characteristic “whooping” sound that results when infected individuals gasp for air typically doesn’t appear until later in the illness.

Children with a persistent cough that lasts more than two weeks, particularly with unstoppable coughing jags, should visit a doctor and get tested, Pinette said.